Method for optimized reference signal downlink transmission in a wireless communication system

ABSTRACT

A method and system optimizes the transmission of a downlink reference signal (DLRS) in a wireless communication system that uses orthogonal division multiple access (OFDMA) for the downlink. Each Node-B (base station) is capable of transmitting the DLRS reference symbols in different subframes of the OFDM radio frame and changing both the number and location of the subframes in response to changing network conditions. The network conditions include the number of terminals being served by the Node-B and multiple access interference (MAI) from adjacent Node-Bs.

RELATED APPLICATION

This application is a continuation of application Ser. No. 11/736,174filed Apr. 17, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to a wireless communication system, likea cellular network, and more particularly to downlink transmission of areference signal in the network.

2. Description of the Related Art

A cellular network is a wireless communication system made up of anumber of cells, each served by a fixed transmitter, known as a cellsite or base station. Each cell site in the network typically overlapsother cell sites. The most common form of cellular network is a mobilephone (cell phone) system. All of the base stations are connected tocellular telephone exchanges or “switches”, which in turn connect to thepublic telephone network or another switch of the cellular company.

The 3^(rd) Generation Partnership Project (3GPP) is a worldwideconsortium to create a specification for a globally applicable thirdgeneration (3G) mobile phone system. 3GPP's plans are currently indevelopment under the title Long Term Evolution (LTE). The 3GPP LTEproject is to improve the Universal Mobile Telecommunications System(UMTS) terrestrial radio access (UTRA) mobile phone standard to copewith future requirements. Goals of 3GPP LTE include improvingefficiency, lowering costs, improving services, making use of newspectrum opportunities, and better integration with other openstandards. The evolved UTRA (E-UTRA) system proposed by 3GPP LTE usesorthogonal frequency division multiple access (OFDMA) for the downlink(base station to mobile phone or terminal) and single carrier frequencydivision multiple access (SC-FDMA) for the uplink (mobile terminal tobase station). In 3GPP LTE terminology, a base station is called a“Node-B” and a mobile terminal is called “user equipment” (UE). The 3GPPLTE technical specification is described in the reference documenttitled 3rd Generation Partnership Project; Technical Specification GroupRadio Access Network; Physical Channels and Modulation (Release 8), 3GPPTS 36.211 V0.4.0 (2007-02).

The 3GPP LTE proposes a multiple-input multiple-output (MIMO) systemwith up to four antennas per base station. This requires that at leastsome of the terminals in the network be capable of receiving not onlybase station transmissions from two antennas (2-TX) but also basestation transmissions from four antennas (4-TX). The base stationstransmit a downlink reference signal (DLRS) that is modulated intoreference symbols that are used by the terminals for channel estimationand measurements. There are two reference symbol structures of interestto 3GPP LTE: one for a 2-TX base station and one for a 4-TX basestation. Thus each base station may be transmitting both referencesymbol structures in each downlink radio frame and at the same locationin each radio frame.

It is desirable to minimize the overhead occupied by the DLRS in thedownlink. Also, the transmission by adjacent Node-Bs of the DLRS maycause multiple access interference (MAI) which will degrade performance.Thus what is needed is a 3GPP LTE wireless communication system thatoptimizes transmission of the DLRS from an individual Node-B as well asfrom adjacent Node-Bs.

SUMMARY OF THE INVENTION

The invention relates to a method and system for optimizing transmissionof the downlink reference signal (DLRS) in a wireless communicationsystem, like the 3^(rd) Generation Partnership Project Long TermEvolution (3GPP LTE) proposed cellular network. Each four-antenna (4-TX)Node-B (base station) in the network is capable of transmitting thereference symbols in different subframes of the OFDM radio frame andchanging both the number and location of the subframes in response tochanging network conditions. The network conditions include the numberof 4-TX terminals being served by the Node-B and multiple accessinterference (MAI) from adjacent Node-Bs.

In one implementation, the reference symbols can be transmitted by eachNode-B in a predefined pattern of subframe locations, with the patternbeing selected from a set of unique patterns. Each pattern has adifferent number of subframes containing the reference symbols. Inresponse to a change in the number of 4-TX capable terminals in theNode-B's cell, the Node-B increases or decreases the number of subframesin which the reference symbols are transmitted by selecting theappropriate one of the unique patterns in the set. The Node-Bs may usethe same set of patterns. Alternatively, each Node-B may use a setdifferent from the sets of other Node-Bs, particularly adjacent Node-Bswhose cells may overlap, to reduce MAI.

The method and system also allows a Node-B to change the location of thesubframes in which the reference symbols are transmitted if an adjacentNode-B is causing MAI in the selected subframes. The terminals use thedownlink reference symbols to measure interference, in the form of asignal-to-interference (SIR) value, and transmit the SIR value in theuplink to the Node-B. In response to the SIR values, the Node-B mayselect a different subframe or subframes in which to transmit thereference symbols to thereby reduce MAI.

For a fuller understanding of the nature and advantages of the presentinvention, reference should be made to the following detaileddescription taken together with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a wireless communication system like thatproposed by 3GPP LTE and shows multiple Node-Bs with overlapping cellsand multiple terminals.

FIG. 2 is an illustration of the generic radio frame structure in thetime domain for the orthogonal frequency division multiplexing (OFDM)downlink.

FIG. 3 is an illustration of the OFDM downlink resource grid andstructure showing a resource block and resource elements within aresource block.

FIG. 4 is an illustration of a radio frame wherein reference symbols R1and R2 are transmitted in each subframe of the radio frame and referencesymbols R3 and R4 are transmitted in alternate subframes of each radioframe.

FIG. 5A is an illustration of a subframe reference symbol (RS) structureA showing the location of reference symbols R1 and R2 in the resourceblock.

FIG. 5B is an illustration of a subframe RS structure B showing thelocation of reference symbols R1, R2, R3 and R4 in the resource block.

FIG. 5C is an illustration of a subframe RS structure C showing thelocation of reference symbol R1 in the resource block.

FIG. 5D is an illustration of a subframe RS structure D showing thelocation of reference symbols R1 and R2 in the resource block.

FIG. 6 is an illustration of a set of four predefined unique patternsfor transmission of subframe RS structures A and B in a radio frame.

FIG. 7 is an illustration of the set of patterns of FIG. 6, but whereinno RS is transmitted in some of the subframes.

FIG. 8 is an illustration of radio frames for three different Node-Bsand shows three unique types of a pattern with four subframe structureBs per radio frame, with each pattern being associated with a Node-B.

FIG. 9A illustrates the location of a subframe RS structure B in a radioframe with intercell interference from an adjacent cell.

FIG. 9B illustrates the location of the subframe RS structure B in theradio frame after repositioning to a subframe with low intercellinterference.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a diagram of a wireless communication system like thatproposed by 3GPP LTE. The system includes a plurality of Node-Bs (basestations) 102, 104, 106; a plurality of UEs (mobile phones orterminals), such as mobile phones or terminals 120, 122, 124; and acentral gateway 130 that provides connection of the system to the publictelephone network. The Node-Bs 102, 104, 106 are connected to thegateway 130 and may be connected to each other.

Each Node-B may serve one or more cells. In the example of FIG. 1,Node-B 102 serves cells 102 a; Node-B 104 serves cells 104 a; and Node-B106 serves cells 106 a. 3GPP LTE proposes a multiple-inputmultiple-output (MIMO) system, so some of the Node-Bs may have multiplereceive antennas and some may have multiple transmit antennas. There areno restrictions on the combinations of Node-B transmit and receiveantennas. In a typical 3GPP LTE proposed network, a Node-B may haveeither two physical transmit antennas (2-TX) or four physical transmitantennas (4-TX). In FIG. 1, Node-B 102 and Node-B 104 are shown as being4-TX Node-Bs and Node-B 106 is shown as being a 2-TX Node-B. Thedownlink refers to transmission from a Node-B to a terminal, and theuplink refers to transmission from a terminal to a Node-B. Some of theterminals may have multiple receive antennas and others may have onlyone receive antenna. In FIG. 1, terminals 120, 122 are depicted ashaving four receive antennas (4-TX) and terminal 124 is depicted ashaving two receive antennas (2-TX). Thus, at any time within anyparticular cell there will typically be some 4-TX terminals capable ofreceiving the downlink from a 4-TX Node-B.

3GPP LTE uses orthogonal frequency division multiple access (OFDMA) forthe downlink. The basic idea underlying orthogonal frequency divisionmultiplexing (OFDM) is the division of the available frequency spectruminto several subcarriers. To obtain a high spectral efficiency, thefrequency responses of the subcarriers are overlapping and orthogonal,hence the name OFDM. In the system of 3GPP LTE, the OFDMA downlinktransmissions and the uplink transmissions are organized into radioframes with T_(f)=307200×T_(s)=10 ms duration. The generic framestructure is applicable to both frequency division duplex (FDD) (theapplication of frequency-division multiplexing to separate outward andreturn signals) and time division duplex (TDD) (the application oftime-division multiplexing to separate outward and return signals). Asshown in FIG. 2, each radio frame is T_(f)=307200×T_(s)=10 ms long andconsists of 20 slots of length T_(slot)=15360×T_(s)=0.5 ms, numberedfrom 0 to 19. A subframe is defined as two consecutive slots wheresubframe i consists of slots 2i and 2i+l. For FDD, 10 subframes areavailable for downlink transmission and 10 subframes are available foruplink transmissions in each 10 ms interval. Uplink and downlinktransmissions are separated in the frequency domain. For TDD, a subframeis either allocated to downlink or uplink transmission. Subframe 0 andsubframe 5 are always allocated for downlink transmission.

The downlink signal in each slot is described by a resource grid of

N_(BW)^(DL)

subcarriers and N_(symb) ^(DL) OFDM symbols. The resource grid andstructure is illustrated in FIG. 3. In case of multi-antennatransmission, there is one resource grid defined per antenna port. Anantenna port is defined by a reference signal, unique within the cell.Each element in the resource grid for an antenna port p is called aresource element and is uniquely identified by the index pair (k, l)where k and/are the indices in the frequency and time domains,respectively. One, two, or four antenna ports are supported. A resourceblock is defined as

N_(symb)^(DL)

consecutive OFDM symbols in the time domain and

N_(BW)^(RB) = 12

consecutive subcarriers in the frequency domain. A resource block thusconsists of

N_(symb)^(DL) × N_(BW)^(RB)

resource elements.

Each Node-B transmits a downlink reference signal (DLRS) that ismodulated into reference symbols in resource blocks. The referencesignal is sometimes called the “pilot” and the reference symbols “pilotinformation.” Since four antenna ports are supported there are fourpossible reference symbols (R1, R2, R3 and R4), with each of the fourreference symbols being associated with an antenna port. The referencesymbols are used by the terminals for channel estimation and physicalmeasurements. Typical measurements that take place within the terminalsinclude signal strength or signal-to-noise ratio (SNR), averagepathloss, and signal-to-interference ratio (SIR) which may berepresented by a channel quality indicator (CQI). These measurements aretransmitted via the uplink back to the Node-Bs. In above, antenna portmeans physical or virtual antenna port.

It may be desirable to use the full Node-B power for 4-TX antennas forcoverage enhancing even when the transmissions are performed using 2-TXantennas. One possibility to achieve this goal is to use precoding tocreate a set of virtual antennas from a set of physical antennas. Thusan antenna may be physical or virtual. The reference signals can also beprecoded using a fixed precoding. The reference signals are thentransmitted over the virtual antennas. It is also possible to create asmaller set of virtual antennas from a larger set of physical antennas.The concept of virtual antennas is described in detail in 3GPP LTEreference document R1-063254, Reference signal structure for 4-TXantenna MIMO, 3GPP TSG RAN WG1 Meeting #47, Riga, Latvia, 6-10 Nov.,2006.

One proposed method of reference signal transmission is as shown in FIG.4, wherein R1 and R2 are transmitted in each subframe of the radio frameand R3 and R4 are transmitted in alternate subframes of each radioframe. Thus two subframe reference symbol (RS) structures arerepresented: subframe RS structure A wherein R1 and R2 are transmitted,and subframe RS structure B wherein R1, R2, R3 and R4 are transmitted.FIG. 5A shows the resource block for subframe structure A, and FIG. 5Bshows the resource block for subframe structure B.

Of course, it is possible to use three or more different subframestructures, for example a subframe structure C wherein only R1 istransmitted, a subframe structure D wherein R1 and R2 are transmitted,and a subframe structure E wherein R1, R2, R3 and R4 are transmitted.FIG. 5C shows the resource block for subframe structure C and FIG. 5Dshows the resource block for subframe structure D. The resource blockfor subframe structure E is identical to the resource block for subframestructure B shown in FIG. 5B. Depending on the subframe structure C, Dor E, the density of reference symbols within the subframe is different,which means that the channel estimation accuracy is different dependingon the subframe structure C, D or E.

It is desirable to minimize the overhead occupied by the DLRS in thedownlink. This invention recognizes that reference symbols R3 and R4,associated with antenna ports 3 and 4, respectively, may not need to betransmitted as frequently as proposed (see FIG. 4), and thus provides amethod to optimize the frequency of transmission of subframe structure Bby increasing or decreasing the number of subframes in which subframestructure B is transmitted.

In one implementation of the invention a set of at least two predefinedunique patterns is available for each Node-B. One set of four patternsis shown in FIG. 6. In pattern 1, subframe structure B occurs only onceper radio frame (B1 at subframe 1). In pattern 2, subframe structure Boccurs twice per radio frame (B1 at subframe 1 and B2 at subframe 5). Inpattern 3, subframe structure B occurs three times per radio frame (B1at subframe 1, B2 at subframe 5, and B3 at subframe 9). In pattern 4,subframe structure B occurs four times per radio frame (B1 at subframe1, B2 at subframe 5, B3 at subframe 9, and B4 at subframe 2). It isunderstood that FIG. 6 is just one example, and thus the number ofpatterns may be more or less than four, and the locations of thesubframe structure Bs within each pattern may be different than in theexample of FIG. 6. It is also within the scope of the invention thatsome of the patterns may have subframes that contain no RS, as depictedin FIG. 7.

In addition to measurement information in the uplink, each terminal alsotransmits capability information to the Node-Bs, which includesidentification of the terminal as a 4-TX MIMO terminal. Thus each Node-Bhas information on the number of 4-TX terminals within its cell at anyone time.

In this invention the number of subframes in which subframe structure Bis transmitted can be varied. For example, the operator of a Node-B mayselect pattern 1 in FIG. 6 when the Node-B first becomes operational,and then if the number of 4-TX terminals increases select pattern 2.Also, the operator may select the appropriate pattern on a regularbasis, for example each day of the week, based on the daily history ofthe number of 4-TX terminals within the Node-B's cell.

In another example, the Node-B may dynamically select the appropriatepattern as the number of 4-TX terminals in the cell changes. Thedecision to switch from one pattern to the next, for example to doublethe frequency of subframe structure B transmission by switching frompattern 1 to pattern 2, would be based on the number of 4-TX terminalsin the cell increasing above a predetermined threshold and remainingabove that threshold for a predetermined period of time.

The set of predefined patterns, like in the examples of FIG. 6 or 7, maybe identical for each Node-B. However, there may be a plurality ofpattern sets, so that most if not all Node-Bs would have a unique set ofpatterns different from the set of other Node-Bs. In particular,physically adjacent Node-Bs which may have overlapping cells, asdepicted in FIG. 1, may have different sets of patterns. In thisimplementation multiple access interference (MAI) from overlapping cellsmay be minimized. FIG. 8 shows an example of three unique types ofpattern 4 (four subframe structure Bs per radio frame) wherein thesubframe locations for the subframe structure Bs are different for eachNode-B. While not shown in FIG. 8, patterns 1, 2 and 3 would also bedifferent for each of the Node-Bs. In 3GPP LTE, each Node-B may have aunique identification, called a “Cell Group ID”. The terminals arecapable of identifying a Node-B by detecting its Cell Group ID. Subframelocations for the subframe structure Bs may be associated with CellGroup IDs. Thus by associating subframe locations for the subframestructure Bs with Cell Group IDs, the terminals can easily find thelocations of subframe structure Bs.

This invention also allows the location of the subframe for subframe RSstructure B to be changed if there is intercell interference, i.e.,signal interference from adjacent Node-Bs. For example, in FIG. 1 the4-TX terminal 120 is located in both cells 102 a and 104 a, but maypresently be served by Node-B 102 and may be subject to signalinterference from Node-B 104 that serves adjacent cell 104 a. Thisaspect of the invention is shown in FIGS. 9A-9B. In FIG. 9A, the 4-TXterminal is in the served cell and the Node-B in that served cell istransmitting one subframe structure B per radio frame at subframe 1. ANode-B in an adjacent cell is transmitting two subframe structure Bs perradio frame at subframes 1 and 6. The terminal measures signalinterference from the adjacent cell for each subframe, in the form ofsignal-to-interference ratio (SIR), and reports that information to theNode-B for the served cell. In the example of FIG. 9A, the terminal hasmeasured high interference in subframe 1 and low interference insubframe 3. In response to the SIR information received in the uplink,the Node-B of the served cell then selects a new subframe in which totransmit subframe structure B. The selection of the new subframe may bemade using a particular algorithm, or simply by selecting the subframewith the lowest interference (highest SIR value). This is shown in FIG.9B where subframe structure B has been repositioned and is nowtransmitted in subframe 3. In this manner MAI is minimized. The SIRinformation transmitted by the terminal may be represented by the CQI,which is measured in the terminal and derived from the reference symbolsas part of the channel estimation process. Other techniques are known tomeasure the SIR in the terminal, for example as described in the 3GPPLTE reference document R1-06-3392, SINR measurements for Scheduling withInterference Coordination, 3GPP TSG RAN WG1 #47 Meeting, Riga, Latvia,Nov. 6-10, 2006.

While the present invention has been particularly shown and describedwith reference to the preferred embodiments, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit and scope of the invention.Accordingly, the disclosed invention is to be considered merely asillustrative and limited in scope only as specified in the appendedclaims.

1. A method for transmitting a reference signal in a cellular networkthat uses orthogonal frequency division multiplexing (OFDM) and includes(a) at least one base station having four antennas (4-TX) in amultiple-input multiple-output (MIMO) system and capable of downlinktransmission of reference symbols representing said four antennas, and(b) a plurality of mobile transmit/receive terminals, each terminalcapable of uplink transmission identifying itself as a 4-TX MIMO capableterminal; wherein the reference symbols are transmitted in OFDM resourceblocks, a resource block comprising a plurality of OFDM subcarriers anda plurality of OFDM symbols in time slots of a radio frame, a radioframe comprising a plurality of subframes, the method comprising:transmitting in the downlink the reference symbols in at least onesubframe per radio frame; receiving at the base station terminalidentification information transmitted in the uplink from saidterminals; and in response to the number of terminals transmitting saidterminal identification information, changing the number of subframesper radio frame in which the reference symbols are transmitted in thedownlink.
 2. The method of claim 1 wherein changing the number ofsubframes comprises selecting one pattern from a set of predefinedunique patterns, each pattern being predefined locations of radiosubframes for the reference symbols.
 3. The method of claim 2 whereinthe set of predefined unique patterns comprises a first pattern with afirst subframe location and a second pattern with said first subframelocation and a second subframe location.
 4. The method of claim 3wherein the set of predefined unique patterns further comprises a thirdpattern with said first and second subframe locations and a thirdsubframe location.
 5. The method of claim 2 wherein said at least onebase station is associated with said set of predefined unique patternsand has a unique cell group identification (ID), and further comprisingtransmitting said cell group ID, whereby a terminal may detect the setof predefined patterns associated with said at least one base station.6. The method of claim 1 wherein the base station is a first basestation and the network includes a second base station transmitting adownlink that interferes with the downlink from the first base station,the method further comprising: determining, from the reference symbolsreceived at a terminal, a value representing a signal-to-interferenceratio (SIR); transmitting the SIR value in the uplink from saidterminal; and in response to the received SIR value, changing thesubframe to a different subframe in which to transmit the referencesymbols in the downlink.